rangeobshash: minor cleanup of the obshash code We achieved minor speedup by delaying the hashing until we know it is needed.

stablerange: remove call to 'repo.revs' It turned out that the percentage of time spend parsing the revset string was two digits, so we compute de heads inline.

stablerange: reuse node calculation as much as possible We have a property cache with that value, so we just reuse it.

stablerange: adds some caching of the subranges The goal of subranges is to be as reusable as possible, so we cache the subrange relationship to take advantage of this.